1. Field of the Invention
The present invention relates to a data communication system suitable for securely transmitting signals (data) indicative of status of a plurality of switches located in a terminal unit to a control unit for controlling operation of a predetermined load.
2. Description of the Prior Art
In recent years, it has been common to control the operation of various kinds of loads, such as a motor and the like, by using a LAN system mounted in an automobile. For example, the activation of a motor for operating a vehicle seat is controlled by tripping the switches mounted in the seat to adjust the position and posture of the seat. In this case, generally, a wire harness is used to connect a control unit for controlling the operation of the predetermined load (motor for operating the seat) with a terminal unit comprising a plurality of switches and sensors. Through the wire harness, switch signals and sensor signals which are detected in the terminal unit are transmitted to the control unit.
Along with an increase of the controlled objects (loads), however, the number of signals (amount of data) in multiplied, which is accompanied by the growth of the lying amount of wire harnesses (the number of signal wires). Given this factor, it is proposed to transmit data (switch signals or sensor signals) from the terminal unit to the control unit through spatial propagation using light signals, such as infrared ray lights or the like, in order to reduce the lying amount of the wire harnesses.
The terminal unit which performs the foregoing optical communication comprises a processing unit (CPU) as main part, he processing unit driving a light-emitting device, such as an infrared-emitting diode or the like, according to the status of a plurality of switches and transmitting light signals of the predetermined frame structure. On the other hand, the control unit comprises an electronic control unit (ECU) as major component, the electronic control unit receiving the light signals by means of a light-receiving device, such as a photo transistor or the like, detecting the switch status, and then controlling the activation of the motor for operating the seat on the basis of the result of the detection.
In the communication system which spatially propagates the light signals, however, there is a fear, for example, that the baggage placed near the seat enters the propagation space of the light signals between the light-emitting device and the light-receiving device. If this happens, the light signals transmitted from the terminal unit is blocked by the baggage (obstacle), so that it is impossible to detect the light signals (switch status) in the control unit. In case that the processing unit (CPU) located in the terminal unit breaks down or that the light-emitting device stops emitting lights, no light signal is output, which makes it impossible for the control unit to detect the switch status.